1. Field of the Invention
The present general inventive concept relates to a registration device to align a leading edge of a paper sheet and an image forming apparatus having the registration device.
2. Description of the Related Art
FIG. 1 is a view illustrating a conventional paper sheet registration method. Referring to FIG. 1, a paper sheet P is picked up from a cassette 5 by a pickup roller 1 and supplied to a registration roller 3. The registration roller 3 conveys the paper sheet P according to a printing speed of a printing element 4. Paper skew may take place in the process of conveying the paper sheet P from the cassette 5 to the registration roller 3. Accordingly, to obtain a printed image with high quality, a registration process is performed to correct the paper skew.
A conventional registration method will now be briefly described with reference to FIG. 1. When the paper sheet P is conveyed to the registration roller 3, the registration roller 3 is rotated in a reverse direction, i.e., a direction opposite to a direction in which the paper sheet P is fed. Thus, when a leading edge of the paper sheet P reaches the registration roller 3, the paper sheet P is prevented from moving further toward the printing element 4. If the pickup roller 1 continues to rotate, the paper sheet P is curled as illustrated by a broken line D in FIG. 1. As a result, the leading edge of the paper sheet P is aligned with respect to the registration roller 3. Subsequently, the registration roller 3 is rotated in a forward direction, to make the paper sheet P move toward the printing element 4.
This method of aligning a paper sheet through the reverse and forward rotation of the registration roller is problematic in the following aspects. First of all, the leading edge of the paper sheet may be damaged when the registration roller rotates in a reverse direction, thereby increasing the likelihood of a paper jam. Second, the registration roller must be stopped during the reverse rotation and then rotated in a forward direction, which is quite difficult due to an inertial resistance of the registration roller. Thus, a motor (not shown) that drives the registration roller may be overloaded. In a case of a high-speed printing image forming apparatus, it is even more difficult to change the rotational direction of the registration roller. Accordingly, it has been proposed to gently reduce the speed of the registration roller in order to stop the reverse rotation, and then to forwardly rotate the registration roller at a gradually increasing speed. However, such deceleration and acceleration process is time-consuming, and therefore reduces the printing speed of the image forming apparatus. Third, a power coupling device of a complicated structure has to be employed in order to rotate the registration roller in both the reverse and forward directions, which increases manufacturing costs of the image forming apparatus.
In addition, an alternate method may be used, wherein the registration roller is stopped temporarily to align the leading edge of the paper sheet, after which the registration roller is rotated forward to convey the paper sheet. However, this method also requires the use of a complicated power coupling device and increase manufacturing costs of the image forming apparatus.